Thursday, December 29, 2016

Trim Wiring

My tail section came with a Ray Allen trim servo for pitch trim, continuing that I recently ordered another trim servo and roll trim bracket. I also recently ordered my stick grips with a 4 way high-hat and now it is time to start planning what wires I will need to put in place to control everything. I think I will be going with a G3X system (to include autopilot) as well as a VP-X electronic circuit breaker controller.

With this mix of electronics it is time to figure out what controls what and what else do I need from a trim and autopilot perspective. My first question was do I need a safety trim? In looking at the G3X system it looks like the preferred approach is to wire the trim servos through the GSA28 autopilot servos. While the VP-X system has safety trim built in, per their install manual, when you install GSA28s with a VP-X you configure the VP-X to disable the variable speed trim functionality. If you dig deeper into what the GSA28s are capable of everything makes a little more sense.

The Garmin servo provides a built-in interface to drive trim servos at no extra cost. When the autopilot is off, the servo provides speed scheduling for the manual trim commands. When the autopilot is on, the servo adjusts to constantly keep the aircraft trimmed. 

What does all of this mean? When the power is removed from the servos, speed scheduling is disabled and the trim commands are passed directly through the servos to the trim motors. Auto trim is not functional when the autopilot is disengaged, and manual electric trim via the stick hat is never disabled, but if you attempt to manually trim the plane when the autopilot is engaged, the autopilot will disengage to honor your request.

It is always your job to trim the plane when the autopilot is disengaged regardless of whether the servos are powered on (and providing speed scheduling) or off and the trim motor drive is passed through without speed scheduling. In general, with these systems, your servos will always be powered, even when you are not using the autopilot.

That seems pretty straight forward and confirmed by the VP-X install manual:

So I started thinking more about that and I got a fairly uncomfortable feeling. Not that I don't trust Garmin, but I don't really trust anyone. I especially don't trust electronics that I did not assemble. If you look at the wiring diagram above you quickly realize that in the event of an autopilot power failure you loose control to your trim. On the RV-10 there is no manual trim wheels so you would be SOL. How can that be? Well... Reading more into the Garmin manual I found this:

In the event that power to the GSA 28 is removed, a fail-safe system connects the trim input switch directly to the trim motor. In this condition, the trim switch powers the trim motor directly and the motor runs at its full speed when the switch is pressed. The same condition also occurs if a trim switch and motor are connected to the GSA 28, but the trim control function is disabled.

Well that makes sense. The next question is how does one wire in the trim switches. I would prefer having both control sticks have full control. I am not sure why other than I like symmetry. In the GSA28 install section you will find a the following note:

So looks like I should be thinking about a GAD 27 as I select my avionics. It is a $500 dollar unit that appears to have a bunch of functionality that I do not need. I went back to the VP-X manual and it looks like there is a trim mixer in that device. So assuming I disable all of the VP-X trim safety systems perhaps I can use the VP-X to mix my trim and G3X to provide auto-trim and speed scheduling. Other than that I still have to check to see if the G3X system has any form of runaway protection.

Tuesday, December 27, 2016

Anti-Rotation Plates

I still have not decided if I am going with a factory new or something like a BPE engine. I did speak with BPE the other day and I told them I am just about to close up my fuel tanks. They told me regardless of my engine choice that it would be a good decision to install fuel return lines into my tank just in case I ever end up with electronic fuel injection. In the case that I go bone stock, I can just cap these connections off, and that seems like some pretty wise advice.

I plan on putting 3/8s AN fitting near my vent line in order to service fuel return. This basically looks just like the vent line, but with a larger fitting to match the flow of the pick-up line. I looked around on Vans for a while, but I did not see an anti-rotation plate for 3/8s fittings. Back to the mill!

There is nothing really earth shattering here. I drilled a couple of #33 holes to have a close fit for some 4-40 screws to hold down the sheet to a scrap delrin block. I will later drill these #33 holes to #30 in order to rivet the plate to the end rib on the tank. From there I milled out a hex pattern to accept the AN fitting.

About the only annoying thing here is that I don't have a CNC. I have gotten fairly decent at running 45 degree lines by hand, but 60 degrees I have yet to master.

PVPV-D Parking Brake Bracket

I want to put a parking brake into the plane. I looked around what others were doing and the Matco PVPV parking brake seems to be pretty standard. So much so that airward sells a bracket for the brake. I have no issues with the airward bracket, but I do have a mill in the basement and this seems like a pretty low stress project for when it is too cold to work in the shop.

I started with building a solidworks model of the PVPV-D from the mechanical drawings. The drawings call out that you need 27.5 degrees of deflection on each side of the "up" position to go from on to off. I built the axial limits on the model stop at 30 degrees on each side.

From there I designed a bracket around the PVPV-D that will:

  • Securely mount the valve to the firewall
  • Allow the replacement of the valve
  • Give 30 degree end stops on the arm travel
  • Hold the control cable

After the design stage I moved over to the mill and faced a hunk of aluminum. I have a TIG welder so I plan to build this in 2 pieces so I don't have to hog out 3 square inches of aluminum. I drilled and tapped the parking PVPV-D mounting holes with a 10-32 tap.

I am still waiting on my fuselage kit to arrive so for the time being I only placed the two valve mounting holes, and none of the firewall mount holes.


At this stage I forgot to keep on taking photos. The second piece I machined was the vertical bracket that provides the end stops and cable holding. I need to purchase a cable yet, so for now, I am going to keep the project in this state until I get the rest of the pieces. I still have a lot of metal to get rid of, but from a proof of concept, the bracket is working.

Friday, December 16, 2016

More Fuel Tanks

Not too much to talk about lately. I have been spending my time working through the fuel tanks. I went back and re-read my last post. I guess a couple of things have changed since then. On the first couple of ribs of the right tank I went with the cleco, cure, and then wet rivet method. I ended up not really liking that method. I started to find the riveting of the ribs while wet quicker and cleaner. I know that sounds counter intuitive given all the complaints about that process, but I like it. It goes quickly, and I do think it pulls the skins to the ribs in a more desirable way.

One of the other interesting things was the right tank fuel sender rod bends. I remember a couple of people complaining that the float hits either the stiffeners or the vent line and that you have to make a couple of bends to get around this. I did not find this to be the case. I started with 3/32 tig rod and made sure I had my bends just right.

After I got everything where I wanted I bent the real rod and checked the clearance and travel. According to the multi-meter I had full travel and there were no clearance issues with the stiffeners or the vent.